The overall purpose of this project is to study mechanisms of protein synthesis, assembly and degradation which play a role in the hypertrophy and atrophy of cardiac muscle. Recently, we reported that calcium can stimulate proteolysis, release of intact myofilaments and to a lesser extent protein synthesis in rat atria and soleus muscles incubated in vitro. In contrast, muscle stretch inhibited proteolysis and myofilament release while stimulating synthesis. In the myocardium, passive tension, in addition to active tension, may be of particular importance in signaling growth since preload (stretching) regulates cardiac output and could, thus, couple compensatory growth to work-load. Additional experiments from our laboratory demonstrated that passive tension stimulates cyclic AMP which, in turn, mediates at least certain of the stretch effects. Using the in vitro atrial and soleus systems we plan to examine the hypothesis that precise combinations of Ca ions and applied tension may be responsible for work-induced cardiac hypertrophy and its regression. More evidence will also be sought supporting a role of myofilament release in the pathway of myofibrillar degradation. Thus, we will quantitate in detail effects of Ca ions, cyclic nucleotides and tension and protein synthesis, proteolysis, and myofilament release while simultaneously measuring active tension. Alteration of intracellular Ca ions will be obtained by varying extracellular Na ion, K ion and Ca ions, and by using Ca ions antagonists such as Verapamil, and with Ca ions ionophores. We will examine the specificity of Ca ions, tension and cyclic nucleotides on the synthesis and degradation of different classes of protein and on the expression of specific myosin isoenzymes characteristic of cardiac hypertrophy. Other experiments will involve measurement of various cyclase and phosphodiesterase activities to determine how tension increases cAMP. Finally, we will analyze by biochemical and electron microscopic techniques changes in the degradative systems in response to Ca ions.